With wide application of various portable electronic equipments and rapid development of an electric automobile, dramatic increase of requirements and performance requirements on their power system-chemical power source, lithium ion batteries are widely applied to the field of mobile electronic terminal equipment with the advantages of large specific energy, high work voltage, low self-discharge efficiency and the like. Furthermore, lithium ion batteries develop towards the direction of higher energy density along with the increase of requirements on a high-specific energy power source. Currently, commercialized lithium ion batteries adopt graphite carbon materials as negative materials in general. It is difficult to obtain breakthrough by adopting an improved battery preparation technology to improve the performance of the battery due to limit of the low theoretical electrochemical capacity (theoretical capacity 372 mAh/g) of the battery. Development of a novel lithium ion battery electrode material with high specific capacity is urgent. Metals such as Si, Sn, Sb and the like are high-capacity negative materials which are much studied by people, wherein the silicon becomes one of preferred negative materials of the next generation of power battery with high specific energy due to the advantages of the theoretical electrochemical capacity (theoretical capacity 4200 mAh/g) which more than 10 times higher than that of the carbon material widely applied at present, low lithiation potential (lower than 0.5V), in existence of common inserting of solvent molecules in the inserting process, abundance in crust and the like. But due to poor conductive performance of the silicon material, and serious volume effect (rate of volume change is 280%-310%) generated during electrochemical lithium insertion and extraction, cracking or crumbling of the material resulted in separation of electrode materials and separation between electrode materials and a current collector, which lead to loss of electric contact, and this structural and electronic degradation thereby leads to fast capacity fading and rapid reduction of the cycle performance of the electrode.
At present, people propose two methods for solving the problem as follows: 1, silicon nanocrystallization, with reduction of particles, the volume change of silicon can be reduced to a certain extent, and internal stress of the electrode is reduced, but the nanosize material is easily aggregated during cycling, it is insufficient to make performance improvement of the practical battery; 2, a nanosize silicon-carbon composite material is adopted, namely nanosize silicon or silicon alloy material with electrochemical activity is inserted or loaded to carbon material. On one hand, the conductive property of the active silicon material can be improved by the carbon material, and on the other hand, the carbon material can be a “buffer skeleton” to disperse and buffer the internal stress of the electrode of the silicon material in the charge and discharge processes caused by volume change, so that the nano silicon-carbon composite material has good cycle stability. Recently, researchers reports (Nature, 2008, 3:31-35) that a silicon nanowire used as negative material for lithium ion batteries not only has the electrochemical capacity close to the theoretical value and good high-rate charge and discharge performances, but also has stable cyclicity. The analysis supports that crystalline silicon in the charge and discharge processes is transformed into amorphous silicon, resulting in phase constituent and structure change of the nanowire due to one-dimensional conduction of electrons in the silicon nanowire along the radial direction, but the one-dimensional structural characteristics are kept invariable, so that good electrical conductivity of the electrode and the stability of the structure are maintained. A crystal structure of the nano silicon is destroyed by lithium ion insertion at normal temperature, so as to a compound of lithium and silicon in a metastable state is generated; and the crystal silicon is transformed into amorphous silicon when lithium is extraction, resulting in volume change, and leading to fading of the battery cycle performance. A research result shows that the amorphous silicon has better capacity retention and cycle performance. On the basis, Cui Y et al (Nano Lett., 2009, 9:3370-3374, WO2010/138617) propose that the silicon nanowire with a core-shell structure of which the core is crystalline silicon and the outer layer is amorphous silicon is prepared on a stainless steel substrate by adopting a chemical vapor deposition method, and used as negative material for lithium ion batteries. The core of the crystalline silicon in the core-shell structure silicon material is used as a skeleton and an electric conductor in the charge and discharge processes; and the amorphous silicon outer layer is using as an active substance for lithium ion insertion and extraction. According to the core-shell structure silicon nanowire negative material, the amorphous silicon can ensure the structure stability in the charge and discharge processes, so that the electric conductivity of the core of the crystalline silicon can not be destroyed. Therefore, the cycle stability of the core-shell structure material is further improved in comparison with the crystalline silicon nanowire. Three-dimensional porous carbon supported nano silicon particles (Nature Materials, 2010, 9:353-358) are prepared by Yushin G and the like by adopting the same method. When the composite structure silicon material is used as the negative material for lithium ion batteries, the three-dimensional porous carbon is taken as the skeleton material, so that on one hand, an effective conductive network can be supplied for the nano silicon particles, and on the other hand, volume expansion of the nano silicon particles also can be buffered by the flexibility of the porous carbon, so that the volume effect of the silicon material in the charge and discharge processes can be inhibited to a certain extent, meanwhile, the electric conductivity of the silicon material is improved, thereby, the cycle stability of the material is improved. Three-dimensional porous carbon-coated silicon composite structure silicon material is prepared by Esmanski A and the like by adopting a template method (Adv. Funct. Mater., 2009, 19:1999-2010). When the material is used as the negative material for lithium ion batteries, the coated carbon can supply a conductive network and buffer the volume effect of nano silicon particles, meanwhile, the three-dimensional porous structure also can contain the volume effect of the silicon material in the charge and discharge cycle process, and has good cycle stability. Thus, we can know that the carbon material mainly plays a role of conducting and buffering the volume expansion of the silicon material in the nano silicon-carbon composite material, the nano silicon material just can develop the electrochemical capacity of the silicon material by effectively combining with the carbon material in the charge and discharge processes, if the nano silicon material falls off from the carbon material in the repeated charge and discharge processes due to expansion and contraction for a plurality of times, the silicon material cannot develop the electrochemical capacity due to loss of the electric contact. A research shows that these nano silicon-carbon composite materials cannot effectively inhibit huge volume effect of the silicon material in a long-term cycle process due to the fact that the nano silicon is deposited on a carbon matrix or the carbon is coated on a nano silicon matrix, and the ductility of the flexible carbon material is limited. Along with cycle, physical combination of the silicon and the carbon becomes worse and worse, resulting in loss of electric contact on separation of the silicon and the carbon material, so that the cycle stability of the material becomes poor, and cannot meet the requirements of the cycle stability of negative material for lithium ion batteries. In conclusion, microstructure of the nano silicon material and effective combination between the nano silicon material and the carbon material are key factors of affecting the performance of the silicon-carbon composite negative material. In addition, the existing methods for preparing these nano silicon-carbon composite materials mainly comprise a chemical vapor deposition method, thermal vapor deposition method, high-temperature pyrolysis, high-energy ball milling and so on. These preparation methods relate complicated technological processes (for example, a template method), or it is difficult to control the process, or the required equipment is expensive (for example, a chemical vapor deposition method), so that it is difficult to achieve scale-up production.